Specific frequency bands for each communications system are defined by cooperation of international and national regulatory bodies. With the increasing number of different communications systems it has become more difficult to define frequency bands dedicated for only one communications system.
Accordingly, it has been envisaged that some of the frequency spectrum may be used by several different communications systems. Authorized Shared Access (ASA) has been discussed in institutional authorities, e.g. CEPT, RSPG and EC. ASA provides a spectrum licensing scheme designed for the controlled secondary usage of the spectrum.
When several communications systems share a frequency band they will cause interference to each other. The amount of interference caused by one system to another depends on the configuration of the system, for example locations of transmitters, the used frequencies and the used transmission powers. This detailed level of information is only available from operators of the communications systems and regarded as critical to their business. Therefore, such information is very difficult to obtain.